Implantable cardiac rhythm management devices are an effective treatment in managing irregular cardiac rhythms in particular patients. Implantable cardiac rhythm management devices are capable of recognizing and treating arrhythmias with a variety of therapies. These therapies range from providing anti-bradycardia pacing for treating bradycardia, anti-tachycardia pacing or cardioversion energy for treating ventricular tachycardia, to high energy shock for treating ventricular fibrillation. Frequently, the cardiac rhythm management device delivers these therapies for the treatment of tachyarrhythmias in sequence; starting with anti-tachycardia pacing and then proceeding to low energy shocks, and then, finally, to high energy shocks. Sometimes, however, only one of these therapies is selected depending upon the tachyarrhythmia detected.
To effectively deliver these treatments, cardiac rhythm management devices must first accurately detect and classify an episode. Through accurate determination and quantification of sensed cardiac events, these cardiac rhythm management devices are able to classify the type of arrhythmia that is occurring and assess the appropriate therapy to provide to the heart, if any. A problem arises, however, when the cardiac rhythm management device senses noise, and mistakenly declares an episode. As a result, in particular instances, the cardiac rhythm management device may inappropriately deliver therapy.
Extra-cardiac noise may cause a cardiac rhythm management device to misclassify noise events as a tachyarrhythmia. In illustration, by incorporating skeletal muscle noise artifact, or other noise, into a cardiac rate calculation, the cardiac rhythm management device might inaccurately calculate the ventricular rate as one that is elevated. If the ventricular rate is mistakenly calculated to be elevated over a threshold rate boundary, a frequent determiner of tachyarrhythmias, the cardiac rhythm management device may inappropriately deliver therapy to a patient.
Additionally, problems arise when the cardiac therapy device withholds therapy after mischaracterizing a sensed event. For example, anti-bradycardia devices deliver a pacing pulse based on whether a cardiac event is sensed within a particular time frame. If the sensing architecture fails to sense a cardiac event within a preset time period, the cardiac rhythm management device will deliver a pacing pulse to the heart. This pacing pulse is timed in a preset sequence to induce the patient's heart to contract in a proper rhythm. This therapy, however, may be compromised by having the cardiac rhythm management device sense and characterize an extraneous event as a “true” cardiac event. If the sensing architecture erroneously classifies noise (such as skeletal muscle artifact or other noise) as a “true” cardiac event, then a pacing pulse may be incorrectly withheld. This is particularly problematic when a pacing pulse is required to maintain a physiologically necessary rate of the patient's heart.
Besides being noticeable and sometimes physically painful to the patient, when a cardiac rhythm management device delivers inappropriate treatment, it can be extremely disconcerting to the patient. Moreover, delivery of an inappropriate therapy can intensify the malignancy of the cardiac arrhythmia. Therefore, the accuracy of a sensing architecture is an important factor in ensuring that appropriate therapy is delivered to a patient.
Current implantable cardiac rhythm management devices incorporate a sensing architecture that detects likely cardiac events and renders a decision regardless of the accuracy of those originally detected events. As such, current implantable cardiac rhythm management devices must include painstakingly designed sensing architectures to try and avoid erroneous detections. Prior art devices have been developed with rudimentary systems and methods in an attempt to determine whether noise is present on a sampled cardiac signal. If noise is detected in these devices, the manner in which the cardiac signal is acquired, or the manner in which the device operates in response to the acquired signal, is altered. This reduces the impact of erroneously detecting noise and, therefore, inappropriately triggering or withholding therapy. This methodology, however, leaves the cardiac rhythm management device open to significant sensing drawbacks, one of which is that it continually perturbates the sensing architecture.
Certain prior art implantable cardiac rhythm management devices continuously adjust parameters such as amplifier gain in response to extra-cardiac noise, which allows for the possibility that the sensing architecture may miss cardiac events. When adjusting the gain control to lessen sensitivity by raising the sensing floor to avoid noise, it is possible to miss actual cardiac events especially during polymorphic rhythms including ventricular fibrillation. In particular, the sensing architecture may miss discrete cardiac beats, or otherwise stated, miss true positives. By missing a cardiac event, rhythm and beat sensitivity is diminished.
Other implantable cardiac rhythm management devices in the prior art repeatedly extend a noise window during continuous noise. When these window extensions either reach a specific number, or more commonly reach the end of a predetermined interval, the device reverts to a non-sensing or asynchronous behavior for a limited period of time. This type of reversion behavior can miss a cardiac event, therefore reducing rhythm and beat sensitivity. Additionally, these reversion approaches to noise are generally only useful for continuous noise. Noise is most frequently burst in nature, for which most reversion schemes are not effective. This often results in overdetection and a potential for inappropriate therapy. Prior art cardiac rhythm management devices frequently utilize these methodologies contiguously.